Image signal processing device, dither pattern generating method, and dither pattern generating program

ABSTRACT

A storage device stores dither patterns composed of three-dimensional block consisting of the number H of dots in a horizontal direction×the number V of lines in a vertical direction×the number F of frame directions. Each value from a minimum value to a maximum value of dither values of n bits is written in each address of the storage device corresponding to each dot. When each value is written into the storage device, processing of obtaining a spatiotemporal density value indicating a degree of density of an address in which a dither value has already been written in a predetermined three-dimensional area centered on each of the addresses in which a new dither value is writable, and processing of selecting an address having the smallest spatiotemporal density value among the addresses in which a new dither value is writable and writing a dither value are repeated.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority under35 U.S.C. § 119 from Japanese Patent Application No. 2018-178444 filedon Sep. 25, 2018, and Japanese Patent Application No. 2018-178452 filedon Sep. 25, 2018, the entire contents of both of which are incorporatedherein by reference.

BACKGROUND

The present disclosure relates to an image signal processing device, adither pattern generating method, and a dither pattern generatingprogram.

An image signal including a first number of gradations by (m+n) bits maybe input to a display that is capable of expressing only a second numberof gradations by m bits, where m and n are predetermined integers. Inthis case, the first number of gradations may be expressed in a pseudomanner by applying n-bit multi-gradation processing to an m-bit imagesignal. One of pseudo multi-gradation processing includes image signalprocessing called frame rate control (FRC) in which the number of bitsis reduced after adding dither data having dither patterns repeating ata plurality of frame periods to an image signal.

SUMMARY

A typical image signal processing device adds different dither patternsof four dots of two horizontal dots and two vertical lines to an imagesignal at four frame periods, and performs pseudo multi-gradationprocessing on the image signal. According to the image signal processingdevice that adds dither data of the dither patterns of four dots at fourframe periods, gradation of 2 bits may be expanded in a pseudo manner.

In order to increase the number of bits to be expanded greater than twobits, the size of a block of a dither pattern is set to be greater thanfour dots and a frame period of adding the dither data of differentdither patterns is set to be longer than four frames. However, whendither data in which one block of a dither pattern is large and a frameperiod is long is added to an image signal, side effects tend to occur.Thus, a dither pattern that is less likely to cause side effects due tothe addition of dither data and that is capable of expanding gradationwith a high quality is required.

A first aspect of one or more embodiments provides an image signalprocessing device including: a storage device configured, when thenumber of dots in a horizontal direction is H, the number of lines in avertical direction is V, and the number of frame directions is F, tostore dither data having a three-dimensional dither pattern composed ofa three-dimensional block in which two-dimensional dither patterns arearranged in the number F in the frame direction, in which the number ofdots of H×V is a number exceeding 4, and each two-dimensional blockconsisting of the number of dots of H×V is set to be one two-dimensionaldither pattern, in which a dither value that is one of n bits is set ineach dot; an adder configured to add a selected dither pattern for eachof the two-dimensional blocks consisting of the number of dots of H×V ina frame of an input image signal having a first number of bits, when thetwo-dimensional dither patterns in the number F in the frame directionare sequentially selected in a frame period F; and a lower bit reductionunit configured to perform limit processing on an overflow at an outputof the adder, and to output an image signal having a second number ofbits obtained by reducing the lower n bits of the first number of bits,wherein each value from a minimum value to a maximum value of dithervalues of n bits is written in each address of the storage devicecorresponding to each dot of the three-dimensional block consisting ofthe number of dots of H×V×F, and each value of the dither values of nbits is assigned to each dot of the three-dimensional block by repeatingthe following processing: a first processing of obtaining the number ofwritten address or addresses in which a dither value has already beenwritten in a predetermined three-dimensional area centered on each oftarget addresses in which a new dither value is writable, when eachvalue of the dither values of n bits is written into the storage device;a second processing of obtaining a spatiotemporal density valueindicating a degree of density of the written addresses in thepredetermined three-dimensional area centered on each of the targetaddresses; a third processing of processing the target address to beless likely to be selected as an address having the smallestspatiotemporal density value, in accordance with the number of writtenaddress or addresses obtained by the first processing; a fourthprocessing of selecting an address having the smallest spatiotemporaldensity value among all of the target addresses after executing thethird processing; and a fifth processing of writing a dither value tothe target address selected as the address having the smallestspatiotemporal density value by the fourth processing.

A second aspect of one or more embodiments provides a dither patterngenerating method of, when the number of dots in a horizontal directionis H, the number of lines in a vertical direction is V, and the numberof frame directions is F, generating dither patterns composed of athree-dimensional block in which two-dimensional dither patterns arearranged in the number F in the frame direction, in which the number ofdots of H×V is a number exceeding 4, and each two-dimensional blockconsisting of the number of dots of H×V is set to be one two-dimensionaldither pattern, in which a dither value that is one of n bits is set ineach dot, the dither pattern generating method including: obtaining thenumber of written address or addresses in which a dither value hasalready been written in a predetermined three-dimensional area centeredon each of target addresses in which a new dither value is writable, ofthe addresses in a storage device corresponding to each dot of thethree-dimensional block consisting of the number of dots of H×V×F;obtaining a spatiotemporal density value indicating a degree of densityof the written addresses in the predetermined three-dimensional areacentered on each of the target addresses; processing each target addressto be less likely to be selected as an address having the smallestspatiotemporal density value, in accordance with the obtained number ofwritten address or addresses; selecting an address having the smallestspatiotemporal density value among all the target addresses, afterprocessing each target address to be less likely to be selected as anaddress having the smallest spatiotemporal density value; writing adither value to the target address selected as the address having thesmallest spatiotemporal density value; storing dither data having thethree-dimensional dither pattern in the storage device, by writing eachvalue from a minimum value to a maximum value of dither values of n bitsin an arbitrary order in each address of the storage devicecorresponding to each dot of the three-dimensional block, by repeatingthe obtaining of the number of written address or addresses, theobtaining of the spatiotemporal density value, the processing of eachtarget address to be less likely to be selected as an address having thesmallest spatiotemporal density value, the selecting of an addresshaving the smallest spatiotemporal density value, and the writing of adither value to the target address selected as the address having thesmallest spatiotemporal density value.

A third aspect of one or more embodiments provides a computer softwareproduct that includes a non-transitory storage medium readable by aprocessor, the non-transitory storage medium having stored thereon a setof instructions for generating dither patterns, when the number of dotsin a horizontal direction is H, the number of lines in a verticaldirection is V, and the number of frame directions is F, generatingdither patterns composed of a three-dimensional block in whichtwo-dimensional dither patterns are arranged in the number F in theframe direction, in which the number of dots of H×V is a numberexceeding 4, and each two-dimensional block consisting of the number ofdots of H×V is set to be one two-dimensional dither pattern, in which adither value that is one of n bits is set in each dot, the instructionsincluding: a first set of instructions of obtaining the number ofwritten address or addresses in which a dither value has already beenwritten in a predetermined three-dimensional area centered on each oftarget addresses in which a new dither value is writable, of theaddresses in a storage device corresponding to each dot of thethree-dimensional block consisting of the number of dots of H×V×F; asecond set of instructions of obtaining a spatiotemporal density valueindicating a degree of density of the written addresses in thepredetermined three-dimensional area centered on each of the targetaddresses; a third set of instructions of processing each target addressto be less likely to be selected as an address having the smallestspatiotemporal density value, in accordance with the number of writtenaddress or addresses obtained by the first set of instructions; a fourthset of instructions of selecting an address having the smallestspatiotemporal density value among all the target addresses afterexecuting the third set of instructions; and a fifth set of instructionsof writing a dither value to the target address selected as the addresshaving the smallest spatiotemporal density value by the fourth set ofinstructions; and a sixth set of instructions of storing dither datahaving the three-dimensional dither pattern in the storage device, bywriting each value from a minimum value to a maximum value of dithervalues of n bits in an arbitrary order in each address of the storagedevice corresponding to each dot of the three-dimensional block, byrepeating the first to fifth sets of instructions.

A fourth aspect of one or more embodiments provides an image signalprocessing device including: a storage device configured, when thenumber of dots in a horizontal direction is H, the number of lines in avertical direction is V, and the number of frame directions is F, tostore dither data having a three-dimensional dither pattern composed ofa three-dimensional block in which two-dimensional dither patterns arearranged in the number F in the frame direction, in which the number ofdots of H×V is a number exceeding 4, and each two-dimensional blockconsisting of the number of dots of H×V is set to be one two-dimensionaldither pattern, in which a dither value that is one of n bits is set ineach dot; an adder configured to add a selected dither pattern for eachof the two-dimensional blocks consisting of the number of dots of H×V ina frame of an input image signal having a first number of bits, when thetwo-dimensional dither patterns in the number F in the frame directionare sequentially selected in a frame period F; and a lower bit reductionunit configured to perform limit processing on an overflow at an outputof the adder, and to output an image signal having a second number ofbits obtained by reducing the lower n bits of the first number of bits,wherein each value from a minimum value to a maximum value of dithervalues of n bits is written in each address of the storage devicecorresponding to each dot of the three-dimensional block consisting ofthe number of dots of H×V×F, when each value of the dither values of nbits is written into the storage device, a processing of obtaining aspatiotemporal density value indicating a degree of density of writtenaddresses in which a dither value has already been written in apredetermined three-dimensional area centered on each of targetaddresses in which a new dither value is writable, and a processing ofselecting an address having the smallest spatiotemporal density valueamong the addresses in which a new dither value is writable and writinga dither value are executed for each two-dimensional block of thethree-dimensional block, each value of the dither values of n bits isassigned to each dot of the three-dimensional block by circulating atwo-dimensional block in which the processing of obtaining aspatiotemporal density value and the processing of selecting an addresshaving the smallest spatiotemporal density value and writing a dithervalue are executed, in the frame direction in an arbitrary order.

A fifth aspect of one or more embodiments provides a dither patterngenerating method of, when the number of dots in a horizontal directionis H, the number of lines in a vertical direction is V, and the numberof frame directions is F, generating dither patterns composed of athree-dimensional block in which two-dimensional dither patterns arearranged in the number F in the frame direction, in which the number ofdots of H×V is a number exceeding 4, and each two-dimensional blockconsisting of the number of dots of H×V is set to be one two-dimensionaldither pattern, in which a dither value that is one of n bits is set ineach dot, the dither pattern generating method including: obtaining aspatiotemporal density value indicating a degree of density of writtenaddresses in which a dither value has already been written in apredetermined three-dimensional area centered on each of targetaddresses in which a new dither value is writable, for eachtwo-dimensional block of the three-dimensional block; selecting anaddress having the smallest spatiotemporal density value among theaddresses in which a new dither value is writable, and writing a dithervalue to the selected address, for each two-dimensional block of thethree-dimensional block; storing dither data having thethree-dimensional dither pattern in the storage device, by writing eachvalue from a minimum value to a maximum value of dither values of n bitsin an arbitrary order in each address of the storage devicecorresponding to each dot of the three-dimensional block, by circulatinga two-dimensional block in which the processing of obtaining aspatiotemporal density value and the processing of selecting an addresshaving the smallest spatiotemporal density value and writing a dithervalue are executed, in the frame direction in an arbitrary order.

A sixth aspect of one or more embodiments provides a computer softwareproduct that includes a non-transitory storage medium readable by aprocessor, the non-transitory storage medium having stored thereon a setof instructions for generating dither patterns, when the number of dotsin a horizontal direction is H, the number of lines in a verticaldirection is V, and the number of frame directions is F, generatingdither patterns composed of a three-dimensional block in whichtwo-dimensional dither patterns are arranged in the number F in theframe direction, in which the number of dots of H×V is a numberexceeding 4, and each two-dimensional block consisting of the number ofdots of H×V is set to be one two-dimensional dither pattern, in which adither value that is one of n bits is set in each dot, the instructionsincluding: a first set of instructions of obtaining a spatiotemporaldensity value indicating a degree of density of written addresses inwhich a dither value has already been written in a predeterminedthree-dimensional area centered on each of target addresses in which anew dither value is writable, for each two-dimensional block of thethree-dimensional block; a second set of instructions of selecting anaddress having the smallest spatiotemporal density value among theaddresses in which a new dither value is writable, and writing a dithervalue to the selected address, for each two-dimensional block of thethree-dimensional block; a third set of instructions of storing ditherdata having the three-dimensional dither pattern in the storage device,by writing each value from a minimum value to a maximum value of dithervalues of n bits in an arbitrary order in each address of the storagedevice corresponding to each dot of the three-dimensional block, bycirculating a two-dimensional block in which the processing of obtaininga spatiotemporal density value and the processing of selecting anaddress having the smallest spatiotemporal density value and writing adither value are executed, in the frame direction in an arbitrary order.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an image signal processing deviceaccording to each embodiment.

FIG. 2 is a diagram illustrating an example of dither patterns of eightframe periods.

FIG. 3 is a flowchart illustrating processes that are premises ofprocesses executed via a dither pattern generating method or ditherpattern generating program according to each embodiment.

FIG. 4 is a diagram conceptually illustrating processes of sequentiallywriting dither values to addresses having the smallest spatiotemporaldensity value in a storage device.

FIG. 5 is a flowchart illustrating processes executed via a ditherpattern generating method or a dither pattern generating programaccording to a first embodiment.

FIG. 6 is a diagram illustrating an example of a three-dimensional areacentered on a target address in first and second embodiments.

FIG. 7 is a flowchart illustrating processes executed via a ditherpattern generating method or a dither pattern generating programaccording to a second embodiment.

FIG. 8 is a diagram illustrating another example of a three-dimensionalarea centered on a target address in a second embodiment.

FIG. 9 is a diagram illustrating an example of a three-dimensional areacentered on a target address in a third embodiment.

FIG. 10 is a flowchart illustrating processes executed via a ditherpattern generating method or a dither pattern generating programaccording to a fourth embodiment.

DETAILED DESCRIPTION

Hereinafter, an image signal processing device, a dither patterngenerating method, and a dither pattern generating program according toeach embodiment will be described with reference to the accompanyingdrawings. The configuration of the image signal processing deviceaccording to each embodiment is common. The configuration and operationof the image signal processing device common to each embodiment will bedescribed with reference to FIG. 1.

In FIG. 1, an image signal processing device according to eachembodiment includes a timing generator 10, a dither pattern generator20, a random access memory (RAM) 30, adders 41 through 43, and lower bitreduction units 51 through 53. As an example, input signals input to theimage signal processing device are an R signal, a G signal, and a Bsignal of 12 bits. The image signal processing device according to eachembodiment outputs an R signal, a G signal, and a B signal of 4 bits byreducing the lower 8 bits after adding a dither pattern described laterto the R signal, the G signal, and the B signal.

The timing generator 10 includes a frame counter 11 configured to countframes based on a vertical synchronization signal, a vertical counterconfigured to count the number of lines in a vertical direction based onthe vertical synchronization signal and a horizontal synchronizationsignal, and a horizontal counter 13 configured to count the number ofdots in a horizontal direction based on the horizontal synchronizationsignal. Note that the vertical counter 12 resets a count value with thevertical synchronization signal and counts up using the horizontalsynchronization signal as a trigger.

A read address of 11 bits, in which the lower 3 bits of a frame countvalue generated by the frame counter 11, the lower 4 bits of a verticalcount value generated by the vertical counter 12, and the lower 4 bitsof a horizontal count value generated by the horizontal counter 13 arecombined, is supplied to the RAM 30. The RAM 30 is an example of astorage device.

The dither pattern generator 20 executes a dither pattern generatingmethod according to each embodiment to generate a dither pattern. Thedither pattern generator 20 may be a central processing unit (CPU) or acomputer that executes a dither pattern generating program according toeach embodiment to generate a dither pattern.

As illustrated in FIG. 2, the dither pattern generator 20 generatesdither patterns of eight frame periods consisting of 256 dots of 16horizontal dots and 16 vertical lines, for example. The dither patternsof eight frame periods will be referred to as dither patterns Dp1through Dp8. The dither patterns Dp1 through Dp8 have different ditherpatterns. The dither patterns Dp1 through Dp8 formed of two-dimensionalblocks are arranged in the frame direction, and a dither patterncomposed of a three-dimensional block is formed by the entire ditherpatterns Dp1 through Dp8.

Each dot of the dither patterns Dp1 through Dp8 may be specified by 2048addresses expressible by 11 bits. In this regard, the dither patterngenerator 20 generates a write address of 11 bits and supplies the writeaddress to the RAM 30. In each embodiment, the number of extended bitsis 8 to reduce an image signal of 12 bits to 4 bits. Thus, the ditherpattern generator 20 generates dither data in which a dither value of 8bits is assigned to each dot of the dither patterns Dp1 through Dp8.That is, the dither value of each dot is any value from 0 to 255.

The RAM 30 includes 2048 addresses and the 2048 addresses correspond toeach dot of the three-dimensional block consisting of the ditherpatterns Dp1 through Dp8. The dither pattern generator 20 generates thedither value of each dot of the dither patterns Dp1 through Dp8, andwrites the respective dither value in an address specified by the writeaddress. Accordingly, the RAM 30 stores the dither data having thedither patterns Dp1 through Dp8 in which the dither value is assigned toeach dot.

When the image signal processing device is activated, the dither patterngenerator 20 generates the dither data having the dither patterns Dp1through Dp8, and writes the dither data into the RAM 30. The dither datastored in the RAM 30 is read by the read address of ii bits and suppliedto the adders 41 through 43.

In FIG. 1, a RAM is used as a storage device for storing the dither datahaving the dither patterns Dp1 through Dp8, but a read-only memory (ROM)in which the dither patterns Dp1 through Dp8 generated by the ditherpattern generator 20 are written in advance may be used. A type ofstorage device is not limited. When a ROM is used as a storage device,the dither pattern generator 20 is provided outside the image signalprocessing device.

The adders 41 through 43 add the dither data of 8 bits to the input Rsignal, G signal, and B signal of 12 bits. A dither pattern of thedither data added to the R signal, G signal, and B signal issequentially selected from the dither patterns Dp1 through Dp8 by theread address. The adders 41 through 43 add the dither data of theselected dither pattern for each block by setting 256 dots of 16horizontal dots and 16 vertical lines in each frame as one block.

The lower bit reduction units 51 through 53 perform limit processing onoverflows of outputs of the adders 41 through 43, respectively, andoutput the R signal, G signal, and B signal of upper 4 bits by reducingthe lower 8 bits.

For example, the lower 8 bits of the R signal, G signal, and B signal of12 bits may be 128 and the added dither data may be any one of 0 to 127.In this case, since addition results by the adders 41 through 43 are 255or less, the addition results do not move up to the upper bits.

The lower bits of the R signal, G signal, and B signal of 12 bits may be128 and the added dither data may be any one of 128 to 255. In thiscase, since the addition results by the adders 41 through 43 are 256 ormore, the addition results move up to the upper bits.

When the frequency of the dither values 0 to 255 of the dither data isuniform, a probability of not being moved up to the upper bits and aprobability of being moved up to the upper bits are half and half.Accordingly, a probability that the lower bit reduction units 51 through53 outputs the original upper 4 bits of the input R signal, G signal,and B signal after reducing 128 of the lower 8 bits, and a probabilityof outputting the upper 4 bits to which +1 is added are half and half.Consequently, 0.5 is expressed on average.

In the above description, the lower 8 bits are 128, but since the lower8 bits are any one of 0 to 255, the lower 8 bits are as followsconsidering all of 0 to 255. Dither data having a dither value of 0 to255 is added to 0 to 255 of the lower 8 bits of the R signal, G signal,and B signal of 12 bits, and the frequency of the lower 8 bits moving tothe upper bit becomes one of 0/256 to 255/256. That is, bit expansion of8 bits is enabled via processing of the adders 41 through 43 and thelower bit reduction units 51 through 53.

The R signal, G signal, and B signal output from the lower bit reductionunits 51 through 53 are 4 bits, but the number of gradations of 12 bitsis expressed in a pseudo manner according to the bit expansion of 8bits.

Next, what patterns of the dither patterns Dp1 through Dp8 are requiredto avoid side effects caused by addition of the dither data and toexpand the gradation with a high quality will be described.

Conditions required for the dither patterns Dp1 through Dp8 include:Condition 1 in which dither values 0 through 255 are distributed asuniformly as possible within one dither pattern; and Condition 2 inwhich dither values in a frame direction respectively at positions ofthe dither patterns Dp1 through Dp8 are distributed as much as possible.

More preferable conditions include, in addition to Conditions 1 and 2:Condition 3 in which a boundary of blocks is not visible within framesof the R signal, G signal, and B signal to which a dither pattern isadded and thus there is almost no visual discomfort at the boundary ofblocks; and Condition 4 in which a boundary of frame periods of ditherpatterns is unlikely visible in a frame direction of the R signal, Gsignal, and B signal to which three-dimensional block composed of thedither patterns Dp1 through Dp8 are added, and periodicity in the framedirection (specifically, flicker disturbance) is unlikely recognized.

A specific generating method for generating the dither patterns Dp1through Dp8 so that at least Conditions 1 and 2 are satisfied will bedescribed with reference to FIGS. 3 and 4. FIGS. 3 and 4 illustrateprocesses that are premises of processes executed via a dither patterngenerating method or a dither pattern generating program according toeach embodiment.

In FIG. 3, the dither pattern generator 20 writes a dither value 0 toall 2048 addresses of the RAM 30 in step S1. The dither patterngenerator 20 resets a counter to set a count value to 0, and sets adither value to 255 in step S2. The dither pattern generator 20calculates a spatiotemporal density value of each address in which adither value is 0 and searches for an address having the smallestspatiotemporal density value in step S3.

The spatiotemporal density value is a value indicating a degree ofdensity of an address in which a dither value has already been writtenin a predetermined three-dimensional area centered on each of theaddresses in which a dither value is newly writable, when a dither valueis to be newly written in the address of the RAM 30. Hereinafter, anaddress in which a dither value has already been written is referred toas a written address. Details about the spatiotemporal density valuewill be described later. The address in which a dither value is newlywritable is an address in which the dither value 0 is written. In theexample illustrated in FIG. 3, since the dither value 0 has already beenwritten in all addresses of the RAM 30, at first, it is possible towrite dither values to all of the addresses, and dither values aresequentially written by repeating the following steps.

The dither pattern generator 20 writes the dither value into the addressof the RAM 30 obtained via step S3 in step S4. In step S4, 255 is firstwritten as the dither value. The dither pattern generator 20 incrementsa count value by 1 in step S5, and determines whether the count value is8 in step S6. If the count value is not 8 (NO), the dither patterngenerator 20 repeats processes of steps S3 through S6. That is, thedither value 255 is written in the RAM 30 in eight addressessequentially obtained in step S3.

If the count value is 8 in step S6 (YES), the dither pattern generator20 resets the counter to set the count value to zero, and decrements thedither value by 1 in step S7. The dither pattern generator 20 determineswhether the dither value is 0 in step S8. If the dither value is not 0(NO), the dither pattern generator 20 repeats the processes of steps S3through S8.

That is, into the RAM 30, the dither value 254 is written eight times,then the dither value 253 is written eight times, and as such,processing of writing the dither value is repeated until the dithervalue 1 is written eight times. Thus, dither data having dither patternscomposed of a three-dimensional block is stored in the RAM 30.

Since the dither value is written in advance, if the dither value is 0in step S8 (YES), the dither pattern generator 20 ends the processing.

Through the above processing, each value of dither values 0 to 255 iswritten eight times to the 2048 addresses of the RAM 30. Since thenumber of addresses of the RAM 30 is 2048 and the number of extensionbits is 8 bits, each value of the dither values 0 to 255 is writteneight times (2048/256) so as to be uniformly assigned to the 2048addresses.

FIG. 4 conceptually illustrates processes of sequentially writing dithervalues to addresses having the smallest spatiotemporal density value. InFIG. 4, the 2048 addresses of the RAM 30 are illustrated in a singledimension. By selecting the address having the smallest spatiotemporaldensity value, an address is selected from an area in athree-dimensionally coarse state in which a written address is notpresent as much as possible, and a new dither value is written.

In FIG. 4, first, eight dither values 255 are written in the RAM 30.Since the eight dither values 255 are written by sequentially selectingthe addresses having the smallest spatiotemporal density value among the2048 addresses, the eight dither values 255 are uniformly distributedwithin one dither pattern and in the frame direction. Note that, in FIG.4, the dither value 0 is written in addresses of a blank portion.

Next, the eight dither values 254 are written in the RAM 30. Similarly,the eight dither values 254 are written by sequentially selecting theaddresses having the smallest spatiotemporal density value among theremaining 2040 addresses, the eight dither values 254 are almostuniformly distributed within one dither pattern and in the framedirection.

Similarly thereafter, each dither value from the dither value 253 to thedither value 1 is written by sequentially selecting the addresses havingthe smallest spatiotemporal density value, among addresses in which adither value is 0 and a new dither value is writable. According to suchprocessing, Conditions 1 and 2 are achieved.

As a comparative example, it is conceivable to randomly select anaddress to which a dither value is to be written by using a pseudorandom number generated by a pseudo random number generator. However,the pseudo random number generator may continuously generate an adjacentaddress or neighboring address, and thus Conditions 1 and 2 are unableto be achieved.

In the example illustrated in FIG. 3, the dither value 0 is written inall 2048 addresses of the RAM 30 in step S1 and each value from thedither value 255 to the dither value 1 is written in a descending order,but this is only an example of the processing. An order of writing eachvalue from the minimum value to the maximum value of the dither value of8 bits in the address of the RAM 30 is arbitrary.

A preferable calculating method of a spatiotemporal density value forachieving Conditions 3 and 4 will be described. The address of the RAM30 is represented by (f, v, h). f indicates a position of a frame of thedither patterns Dp1 through Dp8 and f=0 to 7. v indicates a lineposition of 16 vertical lines and v=0 to 15. h indicates a dot positionof 16 horizontal dots and h=0 to 15.

The dither pattern generator 20 performs filtering processing by athree-dimensional low pass filter (hereinafter, a three-dimensional LPF)with data of written address other than a dither value 0 being 1 anddata of another address being 0. The LPF is a Gaussian filter, forexample. Specifically, the dither pattern generator 20 performs athree-dimensional convolution operation on a kernel function of athree-dimensional LPF and data of an address, based on Equation 1 tocalculate the spatiotemporal density value D (f, v, h).

D(f,v,h)=Σ_(i=−4) ⁴Σ_(j=−8) ⁸Σ_(k=−8) ⁸K(i,j,k)·Q(mod((f+i+8),8),mod((v+j+16),16),mod((h+k+16),16))  (1)

In Equation 1, K (i, j, k) denotes the kernel function of thethree-dimensional LPF. i, j, and k are values for respectivelydetermining a range of a frame direction, a range of a verticaldirection, and a range of a horizontal direction of a three-dimensionalarea centered on an address (f, v, h) at which the spatiotemporaldensity value D(f, v, h) is to be calculated. For example, i=−4 to 4,j=−8 to 8, and k=−8 to 8, and the three-dimensional area may be apredetermined area.

The kernel function K (i, j, k) when a Gaussian filter is used as thethree-dimensional LPF is represented by Equation 2. In Equation 2, σdenotes a standard deviation and a specific value may be a design value.

$\begin{matrix}{{K\left( {i,j,k} \right)} = {\frac{1}{2{\pi\sigma}^{2}}{\exp\left( {- \frac{i^{2} + j^{2} + k^{2}}{2\sigma^{2}}} \right)}}} & (2)\end{matrix}$

Each block of the dither patterns Dp1 through Dp8 is repeatedly used inthe frame and the three-dimensional block of the dither patterns Dp1through Dp8 is repeatedly used in the frame direction. The remainder byb of a is expressed as mod (a, b). Therefore, mod (f+i+8, 8) indicates afirst remainder when (f+i+8) is divided by 8 that is a frame period of adither pattern, mod (v+i+16, 16) indicates a second remainder when(v+i+16) is divided by 16 that is a period (number of lines) in avertical direction, and mod (h+i+16, 16) indicates a third remainderwhen (h+i+16) is divided by 16 that is a period (number of dots) in ahorizontal direction.

Q (f, v, h) is a function (hereinafter, referred to as a function Q)that return 1 when a dither value other than a dither value 0 is writtenin the address (f, v, h) and returns 0 when the address (f, v, h) is inan initial value of the dither value 0. An address obtainable by mod(f+i+8, 8), mod (v+j+16, 16), and mod (h+k+16, 16) is referred to as(f′, v′, h′).

Hence, Q (mod (f+i+8, 8), mod (v+j+16, 16), and mod (h+k+16, 16)) inEquation 1 indicate that 1 is returned when a dither value other thanthe dither value 0 is written in the address (f′, v′, h′) and 0 isreturned when the dither value 0 is written therein.

As such, when the spatiotemporal density value D (f, v, h) is calculatedin each address, 1 or 0 is assigned to each of addresses obtained via aremainder operation using values of (f+i+8), (v+j+16), and (h+k+16)respectively as a frame period, the number of lines, and the number ofdots of dither patterns. Then, the kernel function K (i, j, k) ofthree-dimensional LPF may be multiplied to 1 or 0 of each address toobtain the spatiotemporal density value D (f, v, h). In step S3 of FIG.3, an address having the smallest spatiotemporal density value D (f, v,h) is searched for.

When an address having the smallest spatiotemporal density value issearched for and a dither value is written without using the remainderoperation, the addresses of the upper, lower, left, and right endportions within the frame are likely to be selected as the addresshaving the smallest spatiotemporal density value. Moreover, in the framedirection, the addresses located in the dither pattern Dp1 or Dp8, whichis the end portion in the frame direction, are likely to be selected asthe address having the smallest spatiotemporal density value.

In this case, a boundary of blocks in the frame becomes visible and avisual discomfort is likely to occur at the boundary of blocks. Inaddition, a boundary of a frame period of a three-dimensional blockcomposed of the dither patterns Dp1 through Dp8 becomes visible and thusis easily recognized as flicker disturbance.

By using the remainder operation in the function Q, it is possible toavoid the addresses of the top, bottom, left, and right end portions inthe dither pattern from being easily selected as the address having thesmallest spatiotemporal density value. In addition, it is possible toavoid the address located in the dither pattern at the end portion inthe frame direction from being easily selected as the address having thesmallest spatiotemporal density value. Accordingly, Conditions 3 and 4are achieved.

Meanwhile, i, j, and k that determines a three-dimensional area in whichthe kernel function K (i, j, k) is multiplied to 1 or 0 obtained by thefunction Q are generalized to i=−p to p, j=−q to q, and k=−r to r. p, q,and r are predetermined numbers. The number of frame directions (frameperiod) of a dither pattern is generalized to F, the number of lines ina vertical direction is generalized to V, and the number of dots in ahorizontal direction is generalized to H. F, V, and H are predeterminednumbers. Via such generalization, Equation 1 may be expressed byEquation 3.

D(f,v,h)=Σ_(i=−p) ^(p)Σ_(j=−q) ^(q)Σ_(k=−r) ^(r)K(i,j,k)·Q(mod((f+i+F),F),mod((v+j+V),V),mod((h+k+H),H))  (3)

In each embodiment described above, the number H of dots in thehorizontal direction of the three-dimensional block of the ditherpattern is set to 16, the number V of lines in the vertical direction isset to 16, and the number F of frame directions is set to 8, but are notlimited thereto. The number of dots of H×V of one dither pattern isgreater than 4. It has been confirmed by verification of the inventorthat multi-gradation with less side effects and very high equality isrealized not only when H=16 and V=16, but also when H=32 and V=32.

It has been experimentally confirmed that the number F of framedirections may be 4 to 8 when a frame rate of an image signal is 50 to60 frames per second (fps) and 8 to 16 when the frame rate is 100 to 120fps. The dither pattern generator 20 may be configured such as to changethe number F of frame directions based on the frame rate of the imagesignal. When the image signal processing device illustrated in FIG. 1 isused as a display device capable of changing a frame rate when an imagesignal is displayed, the dither pattern generator 20 may change thenumber F of frame directions based on the frame rate.

When a storage device is configured as a ROM, dither data of the numberF of frame directions corresponding to a plurality of frame rates may bestored in the ROM, or a plurality of ROMs in which dither data of thenumber F of frame directions corresponding to each frame rate is storedmay be provided.

When H=16, V=16, F=8, and the number n of bits (number of extensionbits) of a dither value is 8, the capacity of RAM 30 may be 2048×8 bits.When H=32, V=32, F=8, and n=8, the capacity of RAM 30 may be 8192×8bits. In either case, the capacity of RAM 30 is relatively small.

When H=32, V=32, F=8, and n=8, each value of the dither values 0 to 255is written 32 times from 8192/256 in 8192 addresses of the RAM 30. Instep S6 of FIG. 3, it is determined whether the count value is 32.

By the processes shown in FIG. 3 described above, each dither value isalmost uniformly distributed and written in the RAM 30 in one ditherpattern and in the frame direction. However, in practice, the samedither value may be written to vertically or horizontally adjacentaddresses in one dither pattern. Also, the same dither value may bewritten to adjacent addresses in the frame direction.

Next, the processes of each embodiment will be described, which canreduce the probability that the same dither value will be written toaddresses adjacent in one dither pattern or in the frame direction, andcan distribute each dither value more uniformly in one dither pattern orin the frame direction.

First Embodiment

FIG. 5 illustrates processes in which the dither pattern generator 20generates the dither patterns executing a dither pattern generatingmethod or a dither pattern generating program according to a firstembodiment.

In FIG. 5, the dither pattern generator 20 writes a dither value 0 toall 2048 addresses of the RAM 30 in step S11. The dither patterngenerator 20 resets a counter to set a count value to 0, and sets adither value to 255 in step S12.

In step S13, the dither pattern generator 20 sequentially sets eachaddress of 2048 addresses in the RAM 30 as a target address, and setsthe target address as the excluded address, if a dither value other thanthe dither value 0 is written in the adjacent address of each targetaddress.

As shown in FIG. 6, the adjacent address of the target address is anaddress located to the left, right, above, or below the target addressin each of the dither patterns Dp1 to Dp8, or an address before or afteradjacent to the frame direction. In FIG. 6, an address Atgt surroundedby a thick solid line is an arbitrary target address located in a ditherpattern Dpi which is any one of the dither patterns Dp1 to Dp8. Theleft, right, above, below, before, and after addresses of the targetaddress Atgt shown in FIG. 6 constitute a predeterminedthree-dimensional area centered on the target address Atgt.

The dither pattern generator 20 preferably changes the conditions forsetting the target address as the excluded address according to thenumber of written address or addresses. As an example, when the numberof written address or addresses is equal to or smaller than ⅛ of alladdresses, the dither pattern generator 20 sets the target address asthe excluded address, if a dither value other than the dither value 0 iswritten even in one of the addresses located to the left, right, above,below, before, and after the target address.

In addition, when the number of written addresses is greater than ⅛ andequal to or smaller than ¼ of all addresses, the dither patterngenerator 20 sets the target address as the excluded address, if adither value other than the dither value 0 is written in at least oneset of addresses located to the left and right, above and below, andbefore and after the target address.

When the number of written addresses is greater than ¼ of all addresses,the dither pattern generator 20 does not set the target address as theexcluded address regardless of the state of the adjacent addresses ofthe target address.

In a first embodiment, when the number of target addresses in which adither value has been written is equal to or smaller than a firstnumber, the dither pattern generator 20 excludes the target address fromthe addresses for which the spatiotemporal density value is to beobtained, if the number of written addresses is at least a secondnumber. Here, the first number is ⅛ of all addresses, and the secondnumber is 1.

Moreover, when the number of target addresses in which a dither valuehas been written is greater than the first number and equal to orsmaller than a third number, the dither pattern generator 20 excludesthe target address from the addresses for which the spatiotemporaldensity value is to be obtained, if the number of written addresses isat least a fourth number. Here, the third number is ¼ of all addresses,and the fourth number is 2. Note that the fourth number 2 in a firstembodiment is two addresses symmetrical with respect to the targetaddress.

The dither pattern generator 20 calculates a spatiotemporal densityvalue of each address in which a dither value is 0 and searches for anaddress having the smallest spatiotemporal density value in step S14,except for the excluded addresses set in step S13.

In a first embodiment, if a dither value other than the dither value 0is written in the adjacent address of the target address, since thetarget address is excluded even if the spatiotemporal density value inthe target address in the smallest, the target address will not beselected as the address having the smallest spatiotemporal densityvalue. The process of setting the excluded address in a first embodimentis a process of ultimately processing the target address in which adither value other than the dither value 0 is written in the adjacentaddress to be less likely to be selected as an address having thesmallest spatiotemporal density value.

The dither pattern generator 20 writes the dither value into the addressof the RAM 30 obtained via step S14 in step S15. In step S15, 255 isfirst written as the dither value. The dither pattern generator 20increments a count value by 1 in step S16, and determines whether thecount value is 8 in step S17. If the count value is not 8 (NO), thedither pattern generator 20 repeats processes of steps S13 through S17.

If the count value is 8 in step S17 (YES), the dither pattern generator20 resets the counter to set the count value to zero, and decrements thedither value by 1 in step S18. The dither pattern generator 20determines whether the dither value is 0 in step S19. If the dithervalue is not 0 (NO), the dither pattern generator 20 repeats theprocesses of steps S13 through S19.

If the dither value is 0 in step S19 (YES), the dither pattern generator20 ends the processing.

In FIG. 5, step S13 includes a first process of determining the numberof written address or addresses in a three-dimensional area centered onthe target address. Step S14 includes a second process of obtaining aspatiotemporal density value indicating the degree of density of thewritten addresses in a three-dimensional area centered on the targetaddress. The three-dimensional area in step S13 and thethree-dimensional area in step S14 may not be the same.

Step S13 includes a third process of processing the target address to beless likely to be selected as an address having the smallestspatiotemporal density value, in accordance with the number of writtenaddress or addresses obtained by the first process. Step S14 includes afourth process of selecting an address having the smallestspatiotemporal density value among all of the target addresses afterexecuting the third process. Step S15 is a fifth process of writing adither value to the target address selected as the address having thesmallest spatiotemporal density value.

The dither pattern generator 20 can set the RAM 30 in a state whereeight dither values from the dither value 255 to the dither value 0 arewritten, by executing a sixth process of repeating steps S13 to S19including the first to fifth processes, subsequent to steps S11 and S12.Thus, dither data having a three-dimensional dither pattern is stored inthe RAM 30.

Second Embodiment

FIG. 7 illustrates processes in which the dither pattern generator 20generates the dither patterns executing a dither pattern generatingmethod or a dither pattern generating program according to a secondembodiment.

In FIG. 7, the dither pattern generator 20 writes a dither value 0 toall 2048 addresses of the RAM 30 in step S21. The dither patterngenerator 20 resets a counter to set a count value to 0, and sets adither value to 255 in step S22.

In step S23, the dither pattern generator 20 sequentially sets eachaddress of 2048 addresses in the RAM as a target address, and sets apredetermined coefficient to the target address, if a dither value otherthan the dither value 0 is written in the adjacent address of eachtarget address. As shown in FIG. 6, the adjacent address of the targetaddress is an address located to the left, right, above, or below thetarget address in each of the dither patterns Dp1 to Dp8, or an addressbefore or after adjacent to the frame direction.

As shown in FIG. 8, an address located in a diagonal direction (upperright, lower right, upper left, or lower left) of the target address maybe added to the adjacent address in each of the dither patterns Dp1 toDp8. In FIG. 8, the left, right, above, below, upper right, lower right,upper left, lower left, before, and after the target address Atgtconstitute a predetermined three-dimensional area centered on the targetaddress Atgt.

The dither pattern generator 20 preferably changes the coefficient valueof the coefficient set to the target address according to the number ofwritten address or addresses. As an example, when the number of writtenaddress or addresses is equal to or smaller than ⅛ of all addresses, thedither pattern generator 20 sets a first coefficient having a firstcoefficient value to the target address, if a dither value other thanthe dither value 0 is written even in one of the addresses located tothe left, right, above, below, before, and after the target address. Thefirst coefficient value is a number greater than 1 such as 1.1.

In a case where an address located in the diagonal direction of thetarget address is added to the adjacent address, the dither patterngenerator 20 sets a coefficient to the target address, if a dither valueother than the dither value 0 is written in the diagonally adjacentaddress of the target address. The coefficient value at this time ispreferably a coefficient value smaller than 1.1.

In addition, when the number of written addresses is greater than ⅛ andequal to or smaller than ¼ of all addresses, the dither patterngenerator 20 sets a second coefficient having a second coefficient valueto the target address, if a dither value other than the dither value 0is written in at least one set of addresses located to the left andright, above and below, and before and after the target address. Thesecond coefficient value is a number greater than the first coefficientvalue such as 1.2.

In a case where an address located in the diagonal direction of thetarget address is added to the adjacent address, the dither patterngenerator 20 sets a coefficient to the target address, if a dither valueother than the dither value 0 is written in a set of addresses locatedto the upper right and lower left, or a set of the lower right and upperleft to the target address. The coefficient value at this time ispreferably smaller than 1.2 and greater than 1.1.

When the number of written addresses is greater than ¼ of all addresses,the dither pattern generator 20 does not set a coefficient to the targetaddress regardless of the state of the adjacent addresses of the targetaddress.

In a second embodiment, when the number of target addresses in which adither value has been written is equal to or smaller than a firstnumber, the dither pattern generator 20 sets the first coefficient tothe target addresses, if the number of written addresses is at least asecond number. Here, the first number is ⅛ of all addresses, and thesecond number is 1.

Moreover, when the number of target addresses in which a dither valuehas been written is greater than the first number and equal to orsmaller than a third number, the dither pattern generator 20 sets thesecond coefficient to the target addresses, if the number of writtenaddresses is at least a fourth number. Here, the third number is ¼ ofall addresses, and the fourth number is 2. Note that the fourth number 2in a second embodiment is two addresses symmetrical with respect to thetarget address.

In step S24, the dither pattern generator 20 calculates a spatiotemporaldensity value of each address in which a dither value is 0, multipliesthe calculated spatiotemporal density value by the coefficient in theaddresses where the coefficient is set, and searches for an addresshaving the smallest spatiotemporal density value.

In a second embodiment, if a dither value other than the dither value 0is written in the adjacent address of the target address, a coefficienthaving a coefficient value greater than 1 is set to the target address.As a result, the spatiotemporal density value of the target address ismultiplied by the coefficient value to increase the coefficient value,and the target address is less likely to be selected as an addresshaving the smallest spatiotemporal density value.

The dither pattern generator 20 writes the dither value into the addressof the RAM 30 obtained via step S24 in step S25. The dither patterngenerator 20 increments a count value by 1 in step S26, and determineswhether the count value is 8 in step S27. If the count value is not 8(NO), the dither pattern generator 20 repeats processes of steps S23through S27.

If the count value is 8 in step S27 (YES), the dither pattern generator20 resets the counter to set the count value to zero, and decrements thedither value by 1 in step S28. The dither pattern generator 20determines whether the dither value is 0 in step S29. When the dithervalue is not 0 (NO), the dither pattern generator 20 repeats theprocesses of steps S23 through S29.

If the dither value is 0 in step S29 (YES), the dither pattern generator20 ends the processing.

In FIG. 7, step S23 includes a first process of determining the numberof written address or addresses in a three-dimensional area centered onthe target address. Step S24 includes a second process of obtaining aspatiotemporal density value indicating the degree of density of thewritten addresses in a three-dimensional area centered on the targetaddress. The three-dimensional area in step S23 and thethree-dimensional area in step S24 may not be the same.

Steps S23 and S24 includes a third process of processing the targetaddress to be less likely to be selected as an address having thesmallest spatiotemporal density value, in accordance with the number ofwritten address or addresses obtained by the first process. Step S24includes a fourth process of selecting an address having the smallestspatiotemporal density value among all of the target addresses afterexecuting the third process. Step S25 is a fifth process of writing adither value to the target address selected as the address having thesmallest spatiotemporal density value.

The dither pattern generator 20 can set the RAM 30 in a state whereeight dither values from the dither value 255 to the dither value 0 arewritten, by executing a sixth process of repeating steps S23 to S29including the first to fifth processes subsequent to steps S21 and S22.Thus, dither data having a three-dimensional dither pattern is stored inthe RAM 30.

Third Embodiment

In the above-described first and second embodiments, the adjacentaddresses of the target address is defined as shown in FIG. 6 or 8, butmay be defined as shown in FIG. 9. In FIG. 9, p×p addresses of p in thehorizontal and vertical directions centered on the target address Atgt,and p×p addresses located before and after the target address Atgtadjacent to the frame direction constitute a predeterminedthree-dimensional area centered on the target address Atgt. p is aninteger of 3 or more.

In a third embodiment, as similar to a first embodiment, when the numberof target addresses in which the dither value has been written is equalto or smaller than the first number, the dither pattern generator 20excludes the target address from the addresses for which thespatiotemporal density value is to be obtained, if the number of writtenaddresses is at least a second number. The second number may beappropriately set.

Moreover, when the number of target addresses in which a dither valuehas been written is greater than the first number and equal to orsmaller than the third number, the dither pattern generator 20 excludesthe target address from the addresses for which the spatiotemporaldensity value is to be obtained, if the number of written addresses isat least the fourth number. The fourth number may be appropriately set.

Furthermore, in a third embodiment, as similar to a second embodiment,when the number of target addresses in which the dither value has beenwritten is equal to or smaller than the first number, the dither patterngenerator 20 sets the first coefficient to the target addresses, if thenumber of written addresses is at least a second number. Moreover, whenthe number of target addresses in which a dither value has been writtenis greater than the first number and equal to or smaller than the thirdnumber, the dither pattern generator 20 sets the second coefficient tothe target addresses, if the number of written addresses is at least thefourth number.

The second or fourth number in a third embodiment may not be a set oftwo addresses located at symmetrical positions across the targetaddress. The number of written addresses at any position among the 26addresses located around on the target address may be the second orfourth number.

Next, processing of a fourth embodiment that can reduce the amount ofcalculation in the dither pattern generator 20 will be described.

Fourth Embodiment

In FIG. 10, the dither pattern generator 20 writes a dither value 0 toall 2048 addresses of the RAM 30 in step S41. The dither patterngenerator 20 resets a counter to set a count value to 0, and sets adither value to 255 in step S42.

In step S43, the dither pattern generator 20 calculates a spatiotemporaldensity value of each address in which a dither value is 0, and searchesfor an address having the smallest spatiotemporal density value, in theaddress of the current frame value. The frame value is a count value forselecting any one of the dither patterns Dp1 to Dp8 of eight frameperiods.

The dither pattern generator 20 writes the dither value into the addressof the RAM 30 obtained via step S43 in step S44. The dither patterngenerator 20 increments the frame value by 3 in step S45. The ditherpattern generator 20 decrements the frame value by 8, if the frame valueincremented by 3 is 8 or more.

The dither pattern generator 20 increments the count value by 1 in stepS46, and determines whether the count value is 8 in step S47. If thecount value is not 8 (NO), the dither pattern generator 20 repeats theprocesses of steps S43 through S47.

If the count value is 8 in step S47 (YES), the dither pattern generator20 resets the counter to set the count value to zero, and decrements thedither value by 1 in step S48. The dither pattern generator 20determines whether the dither value is 0 in step S49. If the dithervalue is not 0 (NO), the dither pattern generator 20 repeats theprocesses of steps S43 through S49.

If the dither value is 0 in step S49 (YES), the dither pattern generator20 ends the processing.

According to a fourth embodiment shown in FIG. 10, the process ofsearching for the address with the smallest spatiotemporal density valueand the process of writing the dither value to the address with thesmallest spatiotemporal density value are executed for eachtwo-dimensional block in the three-dimensional block of the addresses ofthe RAM 30. Therefore, the operation amount of the process of searchingfor the address with the smallest spatiotemporal density value is ⅛.

In the example shown in FIG. 10, the frame value circulates in the orderof 0→3→6→1→4→7→2→5→0 . . . over a period of 8 frames. This is taken as afirst circulation example. The method of circulating frame values is notlimited to the first circulation example. As a second circulationexample, the frame value may circulate in the order of0→5→2→7→4→1→6→3→0→ . . . , or as a third circulation example, the framevalue may circulate in the order of 0→1→2→3→4→5→6→7→ . . . . The orderof circulating the two-dimensional block in the frame direction may bean arbitrary order.

According to a fourth embodiment, it is possible to generate a ditherpattern capable of extending gradation with a high quality with a smallamount of calculation, while side effects caused by addition of thedither data is unlikely to occur.

A fourth embodiment may be combined with first to third embodiments. Bycombining fourth embodiment with first to third embodiments, anadvantageous effect to reduce the amount of calculation can be obtained,in addition to the advantageous effect that each dither value can bedispersed more uniformly in one dither pattern and in the framedirection.

The present invention is not limited to first to fourth embodimentsdescribed above and various modifications may be made without departingfrom the scope of the present invention. A first number of bits of aninput image signal and a second number of bits of an output image signalare not limited to 12 bits and 4 bits, respectively, and the number ofextension bits is also not limited to 8 bits.

The configuration illustrated in FIG. 1 may be constituted by hardware(circuit) or software. The use of hardware and software is arbitrary.The dither pattern generating program is stored in a non-transitorystorage medium, loaded in a main memory, and executed by a CPU.

What is claimed is:
 1. An image signal processing device comprising: astorage device configured, when the number of dots in a horizontaldirection is H, the number of lines in a vertical direction is V, andthe number of frame directions is F, to store dither data having athree-dimensional dither pattern composed of a three-dimensional blockin which two-dimensional dither patterns are arranged in the number F inthe frame direction, in which the number of dots of H×V is a numberexceeding 4, and each two-dimensional block consisting of the number ofdots of H×V is set to be one two-dimensional dither pattern, in which adither value that is one of n bits is set in each dot; an adderconfigured to add a selected dither pattern for each of thetwo-dimensional blocks consisting of the number of dots of H×V in aframe of an input image signal having a first number of bits, when thetwo-dimensional dither patterns in the number F in the frame directionare sequentially selected in a frame period F; and a lower bit reductionunit configured to perform limit processing on an overflow at an outputof the adder, and to output an image signal having a second number ofbits obtained by reducing the lower n bits of the first number of bits,wherein each value from a minimum value to a maximum value of dithervalues of n bits is written in each address of the storage devicecorresponding to each dot of the three-dimensional block consisting ofthe number of dots of H×V×F, and each value of the dither values of nbits is assigned to each dot of the three-dimensional block by repeatingthe following processing: a first processing of obtaining the number ofwritten address or addresses in which a dither value has already beenwritten in a predetermined three-dimensional area centered on each oftarget addresses in which a new dither value is writable, when eachvalue of the dither values of n bits is written into the storage device;a second processing of obtaining a spatiotemporal density valueindicating a degree of density of the written addresses in thepredetermined three-dimensional area centered on each of the targetaddresses; a third processing of processing the target address to beless likely to be selected as an address having the smallestspatiotemporal density value, in accordance with the number of writtenaddress or addresses obtained by the first processing; a fourthprocessing of selecting an address having the smallest spatiotemporaldensity value among all the target addresses after executing the thirdprocessing; and a fifth processing of writing a dither value to thetarget address selected as the address having the smallestspatiotemporal density value by the fourth processing.
 2. The imagesignal processing device according to claim 1, wherein the predeterminedarea in the first processing is an area including at least addresseslocated to the left, right, above, and below the target address in thetwo-dimensional dither pattern, and addresses before and after adjacentto the frame direction of the target address.
 3. The image signalprocessing device according to claim 1, wherein the predetermined areais an area including p×p addresses of p in the horizontal direction andp in the vertical direction centered on the target address in thetwo-dimensional dither pattern, and p×p addresses located before andafter the target address adjacent to the frame direction of the targetaddress, where p is an integer of 3 or more.
 4. The image signalprocessing device according to claim 1, wherein the third processing isa processing of excluding the target address from the addresses forwhich the spatiotemporal density value is to be obtained.
 5. The imagesignal processing device according to claim 1, wherein the thirdprocessing is a processing of multiplying the spatiotemporal densityvalue by a coefficient so as to increase the spatiotemporal densityvalue obtained at the target address.
 6. The image signal processingdevice according to claim 4, wherein when the number of the targetaddresses in which a dither value has been written is equal to orsmaller than a first number of all addresses corresponding to each dotof the three-dimensional block, the target address is excluded from theaddresses for which the spatiotemporal density value is to be obtained,if the number of written addresses obtained in the first processing isat least a second number, and when the number of the target addresses inwhich a dither value has been written is greater than the first numberand equal to or smaller than a third number of all addresses, the targetaddress is excluded from the addresses for which the spatiotemporaldensity value is to be obtained, if the number of written addressesobtained in the first processing is at least a fourth number greaterthan the second number.
 7. The image signal processing device accordingto claim 5, wherein when the number of the target addresses in which adither value has been written is equal to or smaller than a first numberof all addresses corresponding to each dot of the three-dimensionalblock, the spatiotemporal density value obtained at the target addressis multiplied by a first coefficient, if the number of written addressesobtained in the first processing is at least a second number, and whenthe number of the target addresses in which a dither value has beenwritten is greater than the first number and equal to or smaller than athird number of all addresses, the spatiotemporal density value obtainedat the target address is multiplied by a second coefficient greater thanthe first coefficient, if the number of written addresses obtained inthe first processing is at least a fourth number greater than the secondnumber.
 8. A dither pattern generating method of, when the number ofdots in a horizontal direction is H, the number of lines in a verticaldirection is V, and the number of frame directions is F, generatingdither patterns composed of a three-dimensional block in whichtwo-dimensional dither patterns are arranged in the number F in theframe direction, in which the number of dots of H×V is a numberexceeding 4, and each two-dimensional block consisting of the number ofdots of H×V is set to be one two-dimensional dither pattern, in which adither value that is one of n bits is set in each dot, the ditherpattern generating method comprising: obtaining the number of writtenaddress or addresses in which a dither value has already been written ina predetermined three-dimensional area centered on each of targetaddresses in which a new dither value is writable, of the addresses in astorage device corresponding to each dot of the three-dimensional blockconsisting of the number of dots of H×V×F; obtaining a spatiotemporaldensity value indicating a degree of density of the written addresses inthe predetermined three-dimensional area centered on each of the targetaddresses; processing each target address to be less likely to beselected as an address having the smallest spatiotemporal density value,in accordance with the obtained number of written address or addresses;selecting an address having the smallest spatiotemporal density valueamong all the target addresses, after processing each target address tobe less likely to be selected as an address having the smallestspatiotemporal density value; writing a dither value to the targetaddress selected as the address having the smallest spatiotemporaldensity value; storing dither data having the three-dimensional ditherpattern in the storage device, by writing each value from a minimumvalue to a maximum value of dither values of n bits in an arbitraryorder in each address of the storage device corresponding to each dot ofthe three-dimensional block, by repeating the obtaining of the number ofwritten address or addresses, the obtaining of the spatiotemporaldensity value, the processing of each target address to be less likelyto be selected as an address having the smallest spatiotemporal densityvalue, the selecting of an address having the smallest spatiotemporaldensity value, and the writing of a dither value to the target addressselected as the address having the smallest spatiotemporal densityvalue.
 9. A computer software product that includes a non-transitorystorage medium readable by a processor, the non-transitory storagemedium having stored thereon a set of instructions for generating ditherpatterns, when the number of dots in a horizontal direction is H, thenumber of lines in a vertical direction is V, and the number of framedirections is F, generating dither patterns composed of athree-dimensional block in which two-dimensional dither patterns arearranged in the number F in the frame direction, in which the number ofdots of H×V is a number exceeding 4, and each two-dimensional blockconsisting of the number of dots of H×V is set to be one two-dimensionaldither pattern, in which a dither value that is one of n bits is set ineach dot, the instructions comprising: a first set of instructions ofobtaining the number of written address or addresses in which a dithervalue has already been written in a predetermined three-dimensional areacentered on each of target addresses in which a new dither value iswritable, of the addresses in a storage device corresponding to each dotof the three-dimensional block consisting of the number of dots ofH×V×F; a second set of instructions of obtaining a spatiotemporaldensity value indicating a degree of density of the written addresses inthe predetermined three-dimensional area centered on each of the targetaddresses; a third set of instructions of processing each target addressto be less likely to be selected as an address having the smallestspatiotemporal density value, in accordance with the number of writtenaddress or addresses obtained by the first set of instructions; a fourthset of instructions of selecting an address having the smallestspatiotemporal density value among all the target addresses afterexecuting the third set of instructions; and a fifth set of instructionsof writing a dither value to the target address selected as the addresshaving the smallest spatiotemporal density value by the fourth set ofinstructions; and a sixth set of instructions of storing dither datahaving the three-dimensional dither pattern in the storage device, bywriting each value from a minimum value to a maximum value of dithervalues of n bits in an arbitrary order in each address of the storagedevice corresponding to each dot of the three-dimensional block, byrepeating the first to fifth sets of instructions.
 10. An image signalprocessing device comprising: a storage device configured, when thenumber of dots in a horizontal direction is H, the number of lines in avertical direction is V, and the number of frame directions is F, tostore dither data having a three-dimensional dither pattern composed ofa three-dimensional block in which two-dimensional dither patterns arearranged in the number F in the frame direction, in which the number ofdots of H×V is a number exceeding 4, and each two-dimensional blockconsisting of the number of dots of H×V is set to be one two-dimensionaldither pattern, in which a dither value that is one of n bits is set ineach dot; an adder configured to add a selected dither pattern for eachof the two-dimensional blocks consisting of the number of dots of H×V ina frame of an input image signal having a first number of bits, when thetwo-dimensional dither patterns in the number F in the frame directionare sequentially selected in a frame period F; and a lower bit reductionunit configured to perform limit processing on an overflow at an outputof the adder, and to output an image signal having a second number ofbits obtained by reducing the lower n bits of the first number of bits,wherein each value from a minimum value to a maximum value of dithervalues of n bits is written in each address of the storage devicecorresponding to each dot of the three-dimensional block consisting ofthe number of dots of H×V×F, when each value of the dither values of nbits is written into the storage device, a processing of obtaining aspatiotemporal density value indicating a degree of density of writtenaddresses in which a dither value has already been written in apredetermined three-dimensional area centered on each of targetaddresses in which a new dither value is writable, and a processing ofselecting an address having the smallest spatiotemporal density valueamong the addresses in which a new dither value is writable and writinga dither value are executed for each two-dimensional block of thethree-dimensional block, each value of the dither values of n bits isassigned to each dot of the three-dimensional block by circulating atwo-dimensional block in which the processing of obtaining aspatiotemporal density value and the processing of selecting an addresshaving the smallest spatiotemporal density value and writing a dithervalue are executed, in the frame direction in an arbitrary order.
 11. Adither pattern generating method of, when the number of dots in ahorizontal direction is H, the number of lines in a vertical directionis V, and the number of frame directions is F, generating ditherpatterns composed of a three-dimensional block in which two-dimensionaldither patterns are arranged in the number F in the frame direction, inwhich the number of dots of H×V is a number exceeding 4, and eachtwo-dimensional block consisting of the number of dots of H×V is set tobe one two-dimensional dither pattern, in which a dither value that isone of n bits is set in each dot, the dither pattern generating methodcomprising: obtaining a spatiotemporal density value indicating a degreeof density of written addresses in which a dither value has already beenwritten in a predetermined three-dimensional area centered on each oftarget addresses in which a new dither value is writable, for eachtwo-dimensional block of the three-dimensional block; selecting anaddress having the smallest spatiotemporal density value among theaddresses in which a new dither value is writable, and writing a dithervalue to the selected address, for each two-dimensional block of thethree-dimensional block; storing dither data having thethree-dimensional dither pattern in the storage device, by writing eachvalue from a minimum value to a maximum value of dither values of n bitsin an arbitrary order in each address of the storage devicecorresponding to each dot of the three-dimensional block, by circulatinga two-dimensional block in which the processing of obtaining aspatiotemporal density value and the processing of selecting an addresshaving the smallest spatiotemporal density value and writing a dithervalue are executed, in the frame direction in an arbitrary order.
 12. Acomputer software product that includes a non-transitory storage mediumreadable by a processor, the non-transitory storage medium having storedthereon a set of instructions for generating dither patterns, when thenumber of dots in a horizontal direction is H, the number of lines in avertical direction is V, and the number of frame directions is F,generating dither patterns composed of a three-dimensional block inwhich two-dimensional dither patterns are arranged in the number F inthe frame direction, in which the number of dots of H×V is a numberexceeding 4, and each two-dimensional block consisting of the number ofdots of H×V is set to be one two-dimensional dither pattern, in which adither value that is one of n bits is set in each dot, the instructionscomprising: a first set of instructions of obtaining a spatiotemporaldensity value indicating a degree of density of written addresses inwhich a dither value has already been written in a predeterminedthree-dimensional area centered on each of target addresses in which anew dither value is writable, for each two-dimensional block of thethree-dimensional block; a second set of instructions of selecting anaddress having the smallest spatiotemporal density value among theaddresses in which a new dither value is writable, and writing a dithervalue to the selected address, for each two-dimensional block of thethree-dimensional block; a third set of instructions of storing ditherdata having the three-dimensional dither pattern in the storage device,by writing each value from a minimum value to a maximum value of dithervalues of n bits in an arbitrary order in each address of the storagedevice corresponding to each dot of the three-dimensional block, bycirculating a two-dimensional block in which the processing of obtaininga spatiotemporal density value and the processing of selecting anaddress having the smallest spatiotemporal density value and writing adither value are executed, in the frame direction in an arbitrary order.